The Evolution of the Hubble TAC Process

Hubble is now entering its 23rd year of operations and its 21st proposal cycle. Over the years, Hubble’s observing capabilities have developed and improved at each servicing mission with the addition of new instrumentation, and with growing expertise in utilizing the facility by the schedulers, the instrument-support scientists, and the community at large. At the same time, the detailed structure of the time allocation process has changed to enable the community to make more effective scientific use of the changing capabilities.

The evolution of the proposal process does not occur in a vacuum. The Institute consults regularly with the Space Telescope User Committee (STUC) and with scientists serving on each time allocation committee (TAC). The Institute has also received advice from ad hoc committees, including the Second Decade Committee and the Space Telescope Institute Council (STIC) TAC Review Committee, and has been happy to profit from the experiences of other observatories.

Cycle 21 will see several additional changes to the Cycle 20 process, and the Call for Proposals outlines those changes. As with any step-by-step process, however, it can be difficult for the community to recall the underlying rationale for each modification. This article gives a brief historical overview, with the aim of describing how the process has changed and giving the context for those changes.

Hubble Proposal Pressure

The pressure from the community to use Hubble has been high throughout most of its history. Figure 1 shows the proposal oversubscription for each cycle; Figure 2 shows the pressure on observing time. The over-subscription level in Cycle 1 was 5:1 by proposals and 8:1 by orbits; the immediately following cycles showed a significant decline with the realization that Hubble’s primary mirror suffered from significant spherical aberration. The Cycle 4 deadline fell in September 1993, before the successful execution of Servicing Mission 1 (SM1) in December of that year, and the uptick of proposals in Cycle 5 clearly reflects the increased confidence of the community in Hubble’s refurbished capabilities. The oversubscription level has risen back to 5:1 in proposals and from 6:1 to 9:1 in orbits in the cycles following the most recent, highly successful, servicing mission, SM4.